[Bug 1877113] Re: call ca_clear_event gives a seg fault

[mdavidsaver via Core-talk <core-talk at aps.anl.gov>]

I tried unsuccessfully to replicate on Linux with the cothread python
client and a soft ioc.

Based on the valgrind message, this isn't as straightforward as *NULL.
Nor does it seem as simple as calling ca_clear_subscription() twice for
the same 'evid'.  However, it might be the case that ca_clear_channel()
has already been called, which invalidates any subscriptions.

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https://bugs.launchpad.net/bugs/1877113

Title:
  call ca_clear_event gives a seg fault

Status in EPICS Base:
  New

Bug description:
  the segmentation fault happends when a client writes to the SYSRESET
  channel of an IOCSTATS - IOC. This kills the IOC and the call of
  ca_clear_event() of SYSRESET on the client gives a this error.

  What can be seen too: is that all connected channels gets a clean
  disconnect except SYSRESET.

  This happends with our 3.14.12 and with 7.0.3

  gdb backtrace of caqtdm(epics 3.14.12):
  #0  0x0000000000000000 in ?? ()
  #1  0x00007fffee1d2f74 in ca_client_context::eliminateExcessiveSendBacklog (this=0x8f8f20, guard=..., chan=...) at ../ca_client_context.cpp:791
  #2  0x00007fffee1d30d1 in eliminateExcessiveSendBacklog (pMon=<value optimized out>) at ../oldAccess.h:482
  #3  ca_clear_subscription (pMon=<value optimized out>) at ../ca_client_context.cpp:761
  #4  0x00007fffe79e80d6 in ClearMonitor (kData=<value optimized out>) at epicsSubs.c:937
  #5  0x00007fffe79e39e5 in Epics3Plugin::pvClearMonitor (this=0x8f74e0, kData=0x7fffffffa600) at epics3_plugin.cpp:86
  #6  0x00007ffff7d79f39 in CaQtDM_Lib::closeEvent (this=0x12016bb0, ce=<value optimized out>) at src/caqtdm_lib.cpp:6445
  #7  0x00000030c67f5cbc in QWidget::event(QEvent*) () from /usr/lib64/libQtGui.so.4
  #8  0x00000030c6b898eb in QMainWindow::event(QEvent*) () from /usr/lib64/libQtGui.so.4
  #9  0x00000030c67aaa0c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib64/libQtGui.so.4
  #10 0x00000030c67b094b in QApplication::notify(QObject*, QEvent*) () from /usr/lib64/libQtGui.so.4
  #11 0x00000030bfb57d0c in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib64/libQtCore.so.4
  #12 0x00000030c67fa711 in QWidgetPrivate::close_helper(QWidgetPrivate::CloseMode) () from /usr/lib64/libQtGui.so.4
  #13 0x00000030c68134b6 in QApplication::x11ClientMessage(QWidget*, _XEvent*, bool) () from /usr/lib64/libQtGui.so.4
  #14 0x00000030c68213c6 in QApplication::x11ProcessEvent(_XEvent*) () from /usr/lib64/libQtGui.so.4
  #15 0x00000030c6849e92 in ?? () from /usr/lib64/libQtGui.so.4
  #16 0x00000030b8a40555 in g_main_context_dispatch () from /lib64/libglib-2.0.so.0
  #17 0x00000030b8a44d08 in ?? () from /lib64/libglib-2.0.so.0
  #18 0x00000030b8a44ec2 in g_main_context_iteration () from /lib64/libglib-2.0.so.0
  #19 0x00000030bfb7d613 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib64/libQtCore.so.4
  #20 0x00000030c6849b5e in ?? () from /usr/lib64/libQtGui.so.4
  #21 0x00000030bfb56752 in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib64/libQtCore.so.4
  #22 0x00000030bfb56a1c in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib64/libQtCore.so.4
  #23 0x00000030bfb58d09 in QCoreApplication::exec() () from /usr/lib64/libQtCore.so.4
  #24 0x000000000040e132 in main ()

  gdb backtrace of caqtdm(epics 7.0.3):
  #0  0x0000000000000000 in ?? ()
  #1  0x00007fffd75df200 in ca_client_context::eliminateExcessiveSendBacklog (this=0x9259c0, guard=..., chan=...) at ../ca_client_context.cpp:785
  #2  0x00007fffd75c2949 in eliminateExcessiveSendBacklog (this=0xc55a98, this=0xc55a98, guard=...) at ../oldAccess.h:481
  #3  ca_clear_channel (pChan=0xc55a98) at ../access.cpp:371
  #4  0x00007fffdc0c590d in ClearMonitor (kData=<optimized out>) at epicsSubs.c:942
  #5  0x00007fffdc0c0a3f in Epics3Plugin::pvClearMonitor (this=0x89ba50, kData=0x7fffffffa700) at epics3_plugin.cpp:87
  #6  0x00007ffff7b4e531 in CaQtDM_Lib::closeEvent (this=0x1049ff0, ce=<optimized out>) at src/caqtdm_lib.cpp:6445
  #7  0x00007ffff5eb5db4 in QWidget::event(QEvent*) () from /lib64/libQtGui.so.4
  #8  0x00007ffff62870d3 in QMainWindow::event(QEvent*) () from /lib64/libQtGui.so.4
  #9  0x00007ffff5e600ec in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /lib64/libQtGui.so.4
  #10 0x00007ffff5e669c5 in QApplication::notify(QObject*, QEvent*) () from /lib64/libQtGui.so.4
  #11 0x00007ffff592e87d in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /lib64/libQtCore.so.4
  #12 0x00007ffff5eb1fd9 in QWidgetPrivate::close_helper(QWidgetPrivate::CloseMode) () from /lib64/libQtGui.so.4
  #13 0x00007ffff5ed0e32 in QApplication::x11ClientMessage(QWidget*, _XEvent*, bool) () from /lib64/libQtGui.so.4
  #14 0x00007ffff5edd98f in QApplication::x11ProcessEvent(_XEvent*) () from /lib64/libQtGui.so.4
  #15 0x00007ffff5f04f34 in x11EventSourceDispatch(_GSource*, int (*)(void*), void*) () from /lib64/libQtGui.so.4
  #16 0x00007ffff1f3f049 in g_main_context_dispatch () from /lib64/libglib-2.0.so.0
  #17 0x00007ffff1f3f3a8 in g_main_context_iterate.isra.19 () from /lib64/libglib-2.0.so.0
  #18 0x00007ffff1f3f45c in g_main_context_iteration () from /lib64/libglib-2.0.so.0
  #19 0x00007ffff595d3ae in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib64/libQtCore.so.4
  #20 0x00007ffff5f050b6 in QGuiEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib64/libQtGui.so.4
  #21 0x00007ffff592d30f in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib64/libQtCore.so.4
  #22 0x00007ffff592d65d in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib64/libQtCore.so.4
  #23 0x00007ffff5932d29 in QCoreApplication::exec() () from /lib64/libQtCore.so.4
  #24 0x000000000040defd in main (argc=4, argv=<optimized out>) at src/caQtDM.cpp:390

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